JP3358482B2 - Lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a lithium secondary battery, and more particularly to an improvement in safety.

[0002]

2. Description of the Related Art With the advance of electronic technology, the performance of electronic equipment has been improved, and its size and portability have been advanced. Conventional secondary batteries include a lead storage battery and a nickel-cadmium battery, but they are still insufficient in terms of high energy density. Therefore, as an alternative to these batteries, lithium secondary batteries with high energy density have been developed,
Spreading rapidly. However, lithium secondary batteries usually use flammable organic solvents for the electrolyte. Therefore, when an operation for raising the battery temperature is performed, for example, when the battery is short-circuited or when the battery is overcharged or dropped into a fire, the electrolyte may burn and the battery may explode violently. For this reason, Japanese Patent Application Laid-Open No. 4-184870 discloses a technique for incorporating a phosphate ester having self-digestibility into an electrolytic solution. Techniques for containing an acid ester compound have been proposed. Further, Japanese Unexamined Patent Publication No.
Japanese Patent No. 45544 proposes a technique in which a chlorine-substituted ester compound is used in an electrolytic solution itself to increase the flash point and ensure safety.

[0003]

However, if the above-mentioned conventional self-digestible substance or a substance having a high flash point is contained in the electrolytic solution, the ionic conductivity of the electrolytic solution is reduced. Discharge characteristics and low-temperature characteristics are reduced.
In addition, even when such a method was used, it was found that the battery exploded when dropped into a fire in a charged state (particularly an overcharged state). That is, even if a technique for making the electrolyte solution nonflammable is employed, it is difficult to prevent a violent explosion when the battery is extremely heated in a charged state. An object of the present invention is to provide a lithium battery that does not reduce the ionic conductivity of the electrolyte and that suppresses a violent explosion even when the battery is exposed to an extremely high temperature in a substantially fully charged state. It is to provide a secondary battery.

[0004]

In order to solve the above problems, the present invention provides a positive electrode and a negative electrode which can be repeatedly charged and discharged.
In a lithium secondary battery having an organic electrolyte, the positive
A material for reducing the positive electrode active material in the electrode, wherein the positive electrode active material
The material that reduces the quality is melted at a predetermined temperature exceeding 60 ° C.
And a reducing agent having a coating layer . Reason can ensure safety by containing the above-mentioned material in the positive electrode, when the reduction of the positive electrode active material in a charged state potential (discharge) proceeds in a direction less noble, because the energy of the positive electrode active material is reduced is there. When the battery is heated, a violent explosion does not occur because the active material has no energy in advance as described above. In addition, since the reducing agent can be appropriately selected from substances that do not dissolve in the electrolytic solution, the ionic conductivity of the electrolytic solution is not reduced unlike the conventional flame retardant. For general working temperature of the battery is -20 ° C. to 60 ° C., a temperature for starting the reduction reaction of the positive electrode active material there is a need a suitable temperature exceeding 60 ° C..

[0005]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 and 2 using a cylindrical lithium ion secondary battery (18650 type) as an example. First, the reducing agent and the oxidizing agent added to the electrode plate will be described. Using tin powder as the reducing agent and iron chloride powder as the oxidizing agent, a coating layer of low-density polyethylene is formed on these surfaces by the following method. A tin powder having a particle size of 5 μm and a low-density polyethylene powder having a particle size of 0.5 μm are charged into a hybridization system manufactured by Nara Machinery. As shown in FIG. 1, in the step of coating the polyethylene, the polyethylene powder 1 is applied to the surface of the tin powder 2, and then a film forming process is performed to form the low-density polyethylene layer 3 on the surface of the tin powder 2. The iron chloride powder can also form the low-density polyethylene layer 3 on the surface of the powder by the above method.

The melting point of this polyethylene is about 115 ° C.
The molecular weight is about 4800. Naturally, the melting point of polyethylene can be changed within a certain range depending on crystallinity, molecular weight, branching, density and the like. The reducing agent and the oxidizing agent added to the electrode plate start to work only when the battery is heated and the low-density polyethylene layer is melted to expose the surfaces of the reducing agent and the oxidizing agent. Therefore, polyethylene having a melting point more suitable for the battery use conditions is properly used.

Next, the fabrication of the positive and negative electrodes will be described. The positive electrode was made of a lithium cobaltate having a diameter of 20 μm, a carbon powder having a diameter of 3 μm, polyvinylidene fluoride, and a reducing agent having the above-described tin powder coated with polyethylene at 80: 5:
A slurry is prepared by adding and mixing N-methylpyrrolidone at a ratio of 5:10 wt%. Then, this slurry is applied to both sides of a 20 μm aluminum foil, dried and rolled,
A positive electrode mixture electrode is prepared, and then cut into a width of 54 mm to form a strip. The negative electrode was composed of a carbon material having a particle diameter of 20 μm, polyvinylidene fluoride, and the above-described iron chloride powder at a ratio of 80: 1.
A slurry is prepared by charging and mixing with N-methylpyrrolidone at a ratio of 0:10 wt%. This slurry is applied to both sides of a 10 μm copper foil, dried and rolled to produce a negative electrode mixture electrode, and then cut into a width of 56 mm to form a strip.

Next, as shown in FIG. 2, the positive electrode 4 and the negative electrode 5 are wound through a separator 6 made of a polyethylene microporous film having a thickness of 25 μm and a width of 58 mm to form a spiral electrode group. The wound electrode group is inserted into a battery container, and a nickel tab terminal that has been welded to the copper foil of the negative electrode current collector in advance is welded to the bottom of the container. Next, 3.7 ml of an electrolyte obtained by dissolving LiPF ₆ at a concentration of 1 mol / l in a solution in which propylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1: 1 was injected into the battery container. Next, the aluminum tab terminal, which was previously welded to the aluminum foil of the positive electrode current collector, was welded to the upper lid, and the upper lid was placed at the top of the battery container via an insulating gasket. did. The initial capacity of the manufactured battery is 1400 mAh.

The reducing agent may be any material that causes an oxidation reaction, such as tin chloride, iron powder, iron sulfate, magnesium, aluminum amalgam, aluminum nickel alloy, lithium aluminum hydride, etc., in addition to the above-mentioned materials.
The oxidizing agent may be any material that causes a reduction reaction, such as silver oxide, copper acetate, copper oxide, manganese dioxide, and lead dioxide, in addition to the above-described materials. In particular, a material that causes a mild reaction is desirable. A material that does not affect the battery reaction and does not cause an oxidation or reduction reaction at 60 ° C. or lower is most preferable. On the other hand, the temperature at which the oxidation and reduction reactions take place is determined by the battery operating temperature. Usually, the reaction may proceed at a temperature exceeding 60 ° C., but when the battery is used in a high-temperature atmosphere, the reaction proceeds at a temperature exceeding 100 ° C. The reaction should be better.

[0010]

EXAMPLE A lithium ion secondary battery (Example 1), a battery containing only a reducing agent in the positive electrode (Example 2), and a oxidizing agent containing only the oxidizing agent in the negative electrode according to the above-described embodiment of the present invention. Battery ( Comparative Example ) and a battery (conventional example) in which the positive and negative electrodes contained neither a reducing agent nor an oxidizing agent were manufactured in the same manner as the manufacturing method described in the embodiment. And charge 1400m
A, 4.2V, 2.5h, discharges the 1400mA, after the capacity check carried out in the cut-off voltage of 2.5V, charging 1400
mA, 4.2V, 3h and 1250mA, 4.4V, 3h
And a projection test (battery drop-in into the fire) according to the UL standard was performed. In each case, the test was performed with 10 batteries, and the results are shown in Table 1. The passed number is shown in the table.

[0011]

[Table 1]

As can be seen from Table 1, Examples 1 and 2
The batteries according to the present invention shown in FIGS.
Unlike the projection test after charging with a normal voltage (4.2 V), not only the projection test after overcharging with a high voltage (4.4 V), but all of the batteries passed, And increase safety. In particular,
To the addition of the reducing agent to the positive electrode that has enhanced safety. With respect to other battery characteristics such as charge / discharge characteristics up to 60 ° C. and cycle characteristics, similar results were obtained between the product of the present invention, the comparative product, and the conventional product.

In the above embodiment, an example in which the reducing agent and the oxidizing agent are covered with polyethylene is shown. However, even if the material is not polyethylene, it is insoluble in the electrolytic solution and can be melted at an appropriate temperature exceeding 60 ° C. It may be coated with any material,
Even if no coating is applied, the reducing agent and the oxidizing agent may be used as long as they have the property of acting as the reducing agent and the oxidizing agent only when the temperature of the reducing agent and the oxidizing agent itself becomes high. It is more preferable that one material has the above characteristics.

[0014]

According to the present invention, a material for reducing a charged positive electrode active material at an appropriate temperature where the battery temperature exceeds 60 ° C. is used as a positive electrode.
Since it is characterized in that it is contained in the battery, it is possible to suppress a violent explosion of a fully charged battery at a high temperature. Even if there is no fully charged battery, the positive electrode active material in a charged state and a constant high temperature since it is possible to guide the pre-discharge condition, is extremely highly safe lithium secondary battery be able to.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of preparation of a reducing agent according to the present invention.

FIG. 2 is an explanatory diagram showing a configuration of a battery according to an example of the present invention.

[Explanation of symbols]

 1. 1. polyethylene powder 2. Tin powder 3. polyethylene layer Positive electrode 5. Negative electrode 6. Separator

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01M 4/62 H01M 4/00-4/04 H01M 10/40

Claims (1)

(57) [Claims] A positive electrode and a negative electrode which can be repeatedly charged and discharged.
In a lithium secondary battery having an organic electrolyte, the positive
A material for reducing the positive electrode active material in the electrode, wherein the positive electrode active material
The material that reduces the quality is melted at a predetermined temperature exceeding 60 ° C.
A lithium secondary battery, characterized in that it is a reducing agent having a coating layer .